It has long been known to employ various means to expand a device within a tube as a way of gripping the tube from within its internal diameter. While there are many examples of this art, one of the more commonly known methods is the plug used in Thermos bottles. In this prior art design, a rubber plug that is slightly smaller than the hole is squeezed between two end restrictions to cause it to expand radially into contact with the hole inside diameter and thus become wedged into the hole.
One application of a tube internal gripper is a grapple attached to the inside of the tubes of a nuclear steam generator. Periodically it is necessary to perform maintenance or inspections from within the heads of these very large heat exchangers. Typically a steam generator has 3,000 to 8,000 tubes originating in a tube sheet above a head that is a hemisphere of five to seven feet radius. When maintenance or inspection is performed, it is necessary to hang equipment such as closed circuit television cameras, pulleys, eddy current test fixtures and other items from the tube sheet. Since the tube sheet surface is usually a layer of 300 series stainless steel, magnetic clamps cannot be used. Furthermore, each of the tubes is welded into the flat tube sheet, so that the tube holes present the only likely surface to which an attachment can be made.
The simple expanding plug, such as that used in a Thermos bottle, is effective for light loads. Since it relies only on friction, it cannot withstand heavy loads if the coefficient of friction is reduced due to wetness or other contaminant on the surface.
The area where such grapple is to be used is highly radioactive. A single worker is permitted to stay in the area for only two or three minutes, during which he is exposed to his three-month radiation dose limit. From this it can be seen that a device than can save even a few seconds in this environment is quite valuable.